DESCRIPTION: The project's goal is to improve clinical dosimetry for photon beam radiotherapy and 192-Ir brachytherapy by developing better primary calibrations for clinical ion chambers. The project has two thrusts which exploit the unique expertise and capabilities in place at the National Research Council. The specific aims of the photon beam sub project are to calibrate a variety of widely used ionization chambers against primary standards of absorbed dose to water in 60-Co and accelerator beams, to use these data to investigate the accuracy of current dosimetry protocols, and to provide data for inclusion in an upcoming AAPM dosimetry protocol. The uncertainty in present dosimetry protocols is >3% (1s). Although the relative uncertainty in similar beams is much better, this large uncertainty applies to doses measured in different institutions and beams e.g. 60-Co and 20 MV. The proposed measurements will reduce the uncertainties to 1% or less. These data are a critical prerequisite to the next generation of dosimetry protocols. The AAPM is developing such a protocol because it is much easier to use than current protocols and would lead to improved accuracy. The specific aim of the 192-Ir sub project is to carry out the basic research necessary to use a cavity chamber as a primary air-kerma standard for high-dose-rate (HDR) 192-Ir sources, which have a mean photon energy of about 350 keV and a range from 10 to 900 keV. For this range of energy, cavity theory can no longer be assumed to hold and free-air-chambers are no longer applicable. There are currently no primary standards for HDR 192-Ir sources and air-kerma calibrations are interpolated from low-energy x-rays to 137-Cs and 60-Co energies using a variety of techniques, all of which are in principle wrong. This sub project will make a primary standard possible by calculating the correction factors needed to account for the breakdown of cavity theory, verifying the calculations by comparison with measurements in low-energy beams and applying them to a standard for 192-Ir based on a cavity ion chamber. The calculations will also be done for the NIST standard chambers to allow the development of an American primary standard for 192-Ir. The air-kerma calibrated 192-Ir sources will also be used to calibrate re-entrant ionization chambers to allow the standards to be disseminated.